This application claims the priority of Japanese Patent Application No. 2001-133247 filed on Apr. 27, 2001, which is incorporated herein by reference.
The present invention relates to a light emitting element using compound semiconductor and a method for manufacturing thereof.
General method for manufacturing light emitting diodes or semiconductor laser elements is such that forming a plurality of compound semiconductor layers on a compound semiconductor singlecrystalline substrate to thereby obtain a multi-layered compound semiconductor wafer having formed therein a p-n junction, and fabricating such wafer into elements. Among these, light emitting diode made from III-V compound semiconductor, and in particular, gallium arsenide phosphide GaAs1-aPa-base (where a relation of 0.45xe2x89xa6axe2x89xa61.0 is satisfied) light emitting diode can be fabricated by forming a plurality of gallium arsenide phosphide GaAs1-aPa (or gallium phosphide GaP) epitaxial layers on a gallium phosphide GaP or gallium arsenide GaAs singlecrystalline substrate, and by diffusing a p-type impurity such as Zn in the uppermost layer of such epitaxial layers, where selecting mixed crystal ratio a will result in light emission at wavelength region covering red, orange and yellow regions. The reason why the range of the mixed crystal ratio a is defined as 0.45xe2x89xa6axe2x89xa61.0 resides in that enhancing indirect transition emission using nitrogen as an isoelectronic trap, which will be described in the next paragraphs.
GaAs1-aPa-base compound semiconductor has a band structure responsible for indirect transition and is not advantageous in obtaining excellent light emission property in the intact state, so that it is a general practice to dope nitrogen (N) to thereby raise the emission efficiency. Nitrogen can raise the emission efficiency since it forms an isoelectronic trap which acts as a luminescent center in the compound semiconductor, to thereby restrict motion of electrons and widen the distribution range of the electron momentum in a wave number vector-momentum space, which successfully increases direct transition components.
Of the entire nitrogen doped in the GaAs1-aPa-base compound semiconductor, only a portion thereof residing in a restricted area in the vicinity of the p-n junction portion, where most of emissive recombination of the carrier occur, is contributable to improve light emission efficiency, whereas excessive nitrogen doped in the other area may adversely lower the emission efficiency since it can act as a photo-absorbing center. Japanese Laid-Open Patent Publication No. 2000-312032 thus proposes a structure of light emitting element in which an area having a nitrogen concentration lower than that in the p-n junction is formed on the p layer side, to thereby suppress the emission loss due to light absorption.
The light emitting element of the foregoing publication however suffers from the problems below.
(1) An extremely wide numerical range of 0.3xc3x971018 atoms/cm3 to 9xc3x971018 atoms/cm3 is disclosed as the concentration of nitrogen to be doped to the p-n junction portion, which is far from being a numerical range for optimizing the emission luminance. There is no consideration on effects on the optimum nitrogen concentration depending on the emission wavelength, that is, exerted by the mixed crystal ratio of a compound semiconductor used.
(2) Too much emphasis on the emission luminance described in the above (1) may not always yield best results in view of long-term sustainment of the element performance.
(3) The improved emission output is ascribable to the constitution in which the n-type layer side of the p-n junction portion is a low-carrier-concentration layer, whereas the p-type layer side has a lowered nitrogen concentration so as to suppress the light absorption. However in such constitution, the light emission is essentially ascribable to hole injection to the n-type layer side, so that the lowered nitrogen concentration in the p-type layer side is not so effective in suppressing the light absorption.
It is therefore a first object of the present invention to provide a GaAsP-base light emitting element capable of sustaining an excellent light emission property for a long period, and a method for manufacturing thereof. It is a second object to provide a GaAsP-base light emitting element having a high luminance and further capable of suppressing light absorption more effectively than in the previous constitution.
To solve the foregoing problems, the light emitting element of the present invention is characterized in that comprising:
a p-n junction portion responsible for light emission formed between a p-type GaAs1-aPa layer (where a represents mixed crystal ratio and satisfies a relation of 0.45xe2x89xa6axe2x89xa61.0) and an n-type GaAs1-aPa layer (where a represents mixed crystal ratio and satisfies a relation of 0.45xe2x89xa6axe2x89xa61.0); and
a first nitrogen-doped zone formed in a portion including the p-n junction interface between such p-type GaAs1-aPa layer and n-type GaAs1-aPa layer,
wherein nitrogen concentration of such first nitrogen-doped zone is set higher than the nitrogen concentration whereat the emission luminance of the light emitting element reaches maximum.
A method for manufacturing a light emitting element is characterized in that manufacturing a light emitting element which comprises a p-n junction portion responsible for light emission formed between a p-type GaAs1-aPa layer (where a represents mixed crystal ratio and satisfies a relation of 0.45xe2x89xa6axe2x89xa61.0) and an n-type GaAs1-aPa layer (where a represents mixed crystal ratio and satisfies a relation of 0.45xe2x89xa6axe2x89xa61.0); and a first nitrogen-doped zone formed in a portion including the p-n junction interface between such p-type GaAs1-aPa layer and n-type GaAs1-aPa layer,
and such method comprises the steps of:
fabricating a plurality of light emitting elements by varying nitrogen concentration Y of the first nitrogen-doped zone while keeping a mixed crystal ratio a of the p-type GaAs1-aPa layer and n-type GaAs1-aPa layer constant;
finding an emission luminance/nitrogen concentration relationship by measuring emission luminance of the individual light emitting elements;
finding from such relationship a nitrogen concentration Yp whereat the emission luminance of the light emitting element will become maximum; and
forming the first nitrogen-doped zone so as to have a nitrogen concentration larger than the nitrogen concentration Yp.
It should now be noted that notation xe2x80x9cGaAs1-aPaxe2x80x9d hereinafter is to express a concept covering both of gallium arsenide phosphide and gallium phosphide unless otherwise being specifically noted.
The light emitting element having formed therein the p-n junction between the n-type GaAs1-aPa layer and p-type GaAs1-aPa layer (referred to as GaAsP-base light emitting element hereinafter, which includes GaP-base light emitting element having mixed crystal ratio a of the n-type GaAs1-aPa layer and p-type GaAs1-aPa layer of 1) will be successful in raising the emission efficiency as described in the above by virtue of doped nitrogen which functions as an isoelectronic trap, but the element is also known to cause gradual decrease in the emission luminance with time of current supply as indicated by curve {circle around (1)} in FIG. 3. As is known from FIG. 3, the luminance decreases with the elapse of cumulative current supply time t from an initial luminance B1 measured immediately after start of the constant current supply. Length of time necessary for the luminance to reach a predetermined limit luminance Bs, or ratio of the initial luminance and luminance attained after the elapse of predetermined time period typically 1,000 hours (referred to as element life L, hereinafter), can provide an index for assessing the element life.
Examination by the present inventors revealed the following facts. As shown in FIG. 2, both of the initial luminance B1 and element life L vary depending on the nitrogen concentration. Of these, the initial luminance Bi reaches maximum at a certain peak nitrogen concentration Yp. The peak nitrogen concentration Yp shifts towards the higher concentration region as the mixed crystal ratio a of GaAs1-aPa increases, that is, the emission wavelength is shortened.
Higher initial luminance Bi will generally be presumed as advantageous in terms of the element life L since a correspondently larger luminance margin will be expected in preparation for the deterioration. That is, it will be a natural way for those skilled in the art to suppose that employing the peak nitrogen concentration Yp whereat the initial luminance Bi reaches maximum undoubtedly optimizes the element life L. Thorough investigations by the present inventors however revealed that such understanding cannot apply to the GaAsP-base light emitting element. As indicated by curve {circle around (1)} in FIG. 3, the peak nitrogen concentration Yp does not always ensures desirable element life L although sufficient emission luminance can be obtained in the initial stage of the current supply. Instead, employing a concentration Cp which resides in a slightly higher concentration range shifted from the peak nitrogen concentration Yp will eminently suppress the time-dependent deterioration in the emission luminance due to prolonged current supply, although the initial luminance Bi becomes Bp which is slightly lower than a value Bmax obtainable at the peak nitrogen concentration Yp as indicated by curve {circle around (2)} in FIG. 3. This successfully results in improvement in the life from Lo to Lp while keeping the initial luminance at a sufficiently high level.
From the investigation results of the present inventors, the nitrogen concentration dependency of the initial luminance Bi and element life L were found to show almost same tendencies irrespective of the mixed crystal ratio a as described in the next. When viewing FIG. 2 along the increasing direction of nitrogen concentration, the initial luminance Bi eminently increases immediately before the peak nitrogen concentration Yp is reached, but moderately decreases thereafter with increase in the nitrogen concentration. On the other hand, the element life L monotonously increases with increase in the nitrogen concentration. So that employing the nitrogen concentration Cp which resides in a range causative of gradual decrease in the initial luminance Bi will successfully extend the element life L without causing a significant decrease in the initial luminance Bi. The nitrogen concentration of the first nitrogen-doped zone is preferably adjusted to 1.05Yp or larger and 1.5Yp or less, where Yp is defined as a peak nitrogen concentration whereat the emission luminance reaches maximum. The nitrogen concentration less than 1.05Yp may result in only an insufficient effect of improving the element life, and exceeding 1.5Yp may result in an extreme shortage of the initial luminance Bi, which may make the element unpractical.
The relation between the initial luminance or element life with nitrogen concentration can be understood assuming a mechanism described in the next. In general, nitrogen atoms capable of acting as luminescent centers in the GaAsP-base light emitting element are only those occupying specific sites (lattice points) in the semiconductor crystal, so that once such sites (referred to as emissive sites, hereinafter) are saturated as shown in FIG. 4C, excessive nitrogen atoms then occupy sites which cannot be responsible for the light emission (referred to as non-emissive sites, hereinafter), which is causative of absorption of the light generated at the emissive sites, to thereby lower the emission efficiency. Thus the emission luminance first increases with increase in the nitrogen concentration until the emissive sites are occupied to a certain extent, and then decreases due to increased light absorption by the nitrogen atoms occupying the non-emissive sites (or a part of emission sites) (which may occasionally be referred to as xe2x80x9cnitrogen-induced auto-absorptionxe2x80x9d, hereinafter). This is a rough explanation for the reason why the nitrogen concentration dependency of the initial luminance Bi has a maximum value (peak nitrogen concentration Yp).
On the other hand, various recent reports suggest a deterioration mechanism by which nitrogen atoms initially occupying the emissive sites gradually migrate to the non-emissive sites in a long duration of current supply to the element (referred to as xe2x80x9cinter-site nitrogen migrationxe2x80x9d, hereinafter). Once such situation occurs, the concentration of nitrogen atoms occupying the emissive sites (referred to as xe2x80x9ceffective nitrogen concentrationxe2x80x9d, hereinafter) gradually decreases despite the apparent nitrogen concentration remains unchanged, which results in the time-dependent degradation of the emission luminance as previously shown in FIG. 3. In this case, as shown in FIG. 4A, the emissive sites are occupied by nitrogen atoms in an approximately exact manner while leaving the non-emissive sites just as true vacant at the peak nitrogen concentration Yp, which provides a condition likely to induce the inter-site nitrogen migration causative of the deterioration. It is thus supposed that, even though the initial luminance Bi is satisfactory, the effective nitrogen concentration will sharply decrease due to promotional trends in the inter-site nitrogen migration, to thereby ruin the element life.
On the contrary, if nitrogen atoms are contained in an amount properly excessive over the peak nitrogen concentration Yp, a part of the non-emissive sites are occupied by the nitrogen atoms already in the initial stage as shown in FIG. 4B, which effectively blocks the nitrogen migration from the emissive sites. The inter-site nitrogen migration is supposed to proceed via the vacant sites (vacant lattice points) similarly to that in general diffusion mechanism. So that the occupied non-emissive sites located in a properly distributed manner will obstruct migration path of the nitrogen atoms, to thereby effectively suppress the inter-site nitrogen migration and improve the element life.
In the present invention, it is allowable to set the carrier concentration of the n-type GaAs1-aPa layer lower than that of the p-type GaAs1-aPa layer, and to form a low-nitrogen-concentration zone, having a nitrogen concentration lower than that of the first nitrogen-doped zone, formed so as to be adjacent to such first nitrogen-doped zone. The light emitting element of the present invention preferably has a p-n junction portion responsible for light emission formed between a p-type GaAs1-aPa layer (where a represents mixed crystal ratio and satisfies a relation of 0.45xe2x89xa6axe2x89xa61.0) and an n-type GaAs1-aPa layer (where a represents mixed crystal ratio and satisfies a relation of 0.45xe2x89xa6axe2x89xa61.0); and has a first nitrogen-doped zone formed in the p-type GaAs1-aPa layer and/or n-type GaAs1-aPa layer so as to include the p-n junction interface, wherein the n-type GaAs1-aPa layer has a carrier concentration lower than that of the p-type of GaAs1-aPa layer, and further preferably has a low-nitrogen-concentration zone, having a nitrogen concentration lower than that of the first nitrogen-doped zone, formed so as to be adjacent to such first nitrogen-doped zone.
In the GaAsP-based light emitting element using the p-n junction, it is preferable in view of improving the emission efficiency to set the carrier concentration of the n-type GaAs1-aPa layer lower than that of the p-type GaAs1-aPa layer, where both layers compose the p-n junction. More specifically, the majority carrier concentration of the n-type layer is decreased to as low as 0.1xc3x971015/cm3 to 3.0xc3x971015/cm3, and the majority carrier concentration of the p-type layer is raised to as high as 1.0xc3x971018/cm3 to 5.0xc3x971018/cm3, to thereby attain a structure in which hole injection into the n-type layer based on the carrier concentration gradient at the junction is prevailing. In the case of employing such structure, nitrogen-induced auto-absorption loss of light can effectively be suppressed and the emission efficiency of the element can be improved, if the first nitrogen-doped zone having a relatively high nitrogen concentration is formed in the n-type layer specifically within a range distant by a predetermined length away from the p-n junction, and using the residual portion of the n-type layer adjacent thereto as a low-nitrogen-concentration zone.
In the above constitution, also the p-type GaAs1-aPa layer can have formed therein the low-nitrogen-concentration zone, having a nitrogen concentration lower than that of the first nitrogen-doped zone, so as to be adjacent to such first nitrogen-doped zone. The low-nitrogen-concentration zone formed in the p-type layer similarly show an effect of improving the emission efficiency of the element although not so eminent as the effect shown by that on the n-type layer side.
While an optimum range of the nitrogen concentration of the first nitrogen-doped zone may vary depending on the mixed crystal ratio a of the n-type GaAs1-aPa layer and the p-type GaAs1-aPa layer as described in the above, it will never be set outside a range from 1.33xc3x971018 atoms/cm3 to 5.56xc3x971018 atoms/cm3. The nitrogen concentration of less than 1.33xc3x971018 atoms/cm3 may result in considerably lowered emission intensity due to shortage of the isoelectronic trap sites, and the concentration exceeding 5.56xc3x971018 atoms/cm3 may again result in lowered emission intensity due to increased auto-absorption loss of light induced by nitrogen.
On the other hand, the low-nitrogen-concentration zone is preferably provided as a second nitrogen-doped zone having a nitrogen concentration within a range from 1.06xc3x971018 atoms/cm3 to 5.28xc3x971018 atoms/cm3. Since there is still some probability of causing emissive recombination also in the low-nitrogen-concentration zone distant from the p-n junction, so that doping of nitrogen also in such zone in an amount of 1.06xc3x971018 atoms/cm3 or above can further raise the emission efficiency. Doping of nitrogen exceeding 5.28xc3x971018 atoms/cm3 will however result in lowered emission intensity due to increased auto-absorption loss of light induced by nitrogen.